Electric signalling systems



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ATTORNEY March 28, 1961 H. K. ROBIN ELECTRIC SIGNALLING SYSTEMS Original Filed Dec. 11, 1950 9 Sheets-Sheet 9 ATTORNEY United States Patent 2,977,416 ELECTRIC SIGNALLING SYSTEMS Original application Dec. 11, 1950, Ser. No. 200,150,

now Patent No. 2,816,163, dated Dec. 10, 1957. Di-

vided and this application Mar. 28, 1957, Ser. No. 649,138

Claims priority, application Great Britain Dec. 14, 1949 24 Claims. (Cl. 178- 51) The present invention relates to electric signalling systems of the type in which different items of information to be transmitted in succession are represented respectively by different selections of pulses from a group of pulses. An item of information to be transmitted may be represented by the selection of one or more of the pulses, or by the selection of no pulses, in the group.

Such systems include, for example, teleprinter systems in which different characters to be transmitted are represented by different selections of pulses from a group of usually five pulses. A further example is a pulse code modulation system in which samples of the amplitude of a wave are represented by the selection of pulses from a group of, say, five pulses, different amplitude levels being represented by difierent selections of pulses.

It is an object of the present invention to provide an improved system of the type specified, particularly, but not exclusively, for use with long distance radio links.

A further object of the invention is to provide an improved system of the type specified adapted to provide a plurality of communication channels on a time sharing basis.

According to the present invention, at a transmitterin an electric signalling system of the type specified, means are provided responsive to each of the said selections to generate a burst of oscillations, the frequencies of the oscillations in successive bursts being determined by Suecessive selections respectively. The group may contain, for example, five pulses representing the five digits of a five-digit binary series. The thirty-two possible selections of pulses are then represented by thirty-two different frequencies respectively.

The said responsive means preferably comprise a plurality of sources of oscillations of different frequencies connected through gate devices respectively to a common output terminal, and control apparatus whereby only one of the gate devices opens in response to a selection from the said group.

The control apparatus preferably comprises a rectifier matrix having a number of input control circuits equal to the total number of pulses in the said group, and a plurality of output terminals connected to the said gate devices respectively to control the opening and closing thereof, the input control circuits being responsive to the said selections to apply control voltages to the matrix, and the matrix being arranged in such a manner that only one of the gates is opened thereby at any instant irrespective of the voltages applied to the matrix from the input control circuits.

In a preferred embodiment of the invention a plurality of sources each providing items of information in the form of selections of pulses from a group are adapted to be connected to the said responsive means in turn through a distributor whereby multi-channel communication can be established. The several sources of infor mation may each be in the form of an automatic sender 7 2,977,416. Patented Mar. 28, 1961 tained by a selection from a group of five pulses. The senders are synchronized to generate signals in turn and a distributor is provided to apply the successively-generated teleprinted signals to the said responsive means.

According to the invention each item of information is presented in a one-out-of-m pulse code, that is, each item of information is represented by a single predetermined pulse selection in a pulse code of in possible selections. In the example given herein the selection is made from a 5-pulse code where 112 has a value of 32.

Further, according to the invention, a receiver for receiving the bursts of transmitted oscillations from a transmitter as set forth above comprises a demodulator, apparatus for converting each demodulated burst of oscillations into a group of pulses having characteristics identified with the said selections. Such apparatus preferably comprises means for applying at least a predetermined fraction of each burst to a counting device for counting the number of cycles of oscillation in the burst. The counting device is preferably a binary counter whereby a number of output pulses are provided in dependence upon the number of cycles counted when the system is used for transmitting teleprinter characters converted into signals of a form suitable for operating a teleprinter. Where the system is used to provide a plurality of communication channels the output of the receiver is passed through a distributor.

This application is a division of my earlier filed ap plication Serial No. 280,150, filed December 11, 1950, now Patent No. 2,816,163, dated December 10, 1957.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1, which has been subdivided and does not appear as a single figure, is a schematic diagram of apparatus at a transmitting terminal. It is convenient to divide Figure 1 into four parts, Figures 1(a) to 1(d), of which Figure 1(a) shows an arrangement of 1S synchronised automatic senders for generating teleprinter signals in turn.

Figure 1(b) is a diagrammatic sketch of a generator for generating 32 oscillations of different frequencies;

Figure 1(0) is a theoretical circuit diagram of control apparatus for controlling the transmission of oscillations generated by the arrangement of Figure 1(b) in dependence upon signals generated by the automatic senders shown in Figure 1(a) and for generating teleprintedsignals, each signal being ob Figure 1(d) is a block schematic diagram of a transmitter for transmitting the oscillations selected by the arrangement of Figure 1(c).

Figure 2 is a schematic diagram of a receiving terminal. It is convenient to divide Figure 2 into nine parts-Figures 2(a) to 2(i)of which Figures 2(a), (b), (c), (d), and (e) are block schematic diagrams of parts of a receiver for use in receiving signals transmitted by the arrangement of Figure 1;

Figures 2(i), (g) and (It) show parts of a distributor for distributing the output of the arrangement shown in Figure 2(a) to 15 channels and for providing in each channel voltages of a form suitable for operating a teleprinter, and

Figure 2(i) is a block diagram of a preferred form of synchronising circuit.

Figure 3 is a diagram illustrating the wave form of a teleprinter signal.

Figure 1(a) shows fifteen automatic senders A to A of known kind, each having five packers (not shown), a pecker bar (not shown) and an operating coil, the fifteen operating coils being shown at 1. to L respectively. These senders may each be a Western Union Type 22A Tape Transmitter. The intelligence to be transmitted is in the form of sets of coded perforations in fifteen, five-unit, standard, perforated tapes, which are passed through the senders A to A respectively. The senders are of the notched-on type, that is to say,

each time the operating coil of a sender is energised the tape in the sender is moved a predetermined distance (the distance between adjacent sets of coded holes in the tape) by a toothed wheel whose teeth engage perrations in the tape additional to the coded perforations.

The toothed wheel is arranged to rotate through a suitable angle each time the operating coil is energised. Each time the tape comes to rest, one or more .peckers make contact with the pecker bar depending upon thecode of the perforations in the tape.

One terrnbinal of each of the coils L to L is con- .nected to one terminal of a battery E whose other terminal is earthed, and the other terminals of the coils L to L are connected to fixed contacts 1 to 15 re- ;spectively of a distributor D whose moving contact 16 is ,earthed. Rotation of the contact arm 16 causes the coils L to L to be energised in turn and hence causes the tapes in the senders A to A to be notched-on in turn. The moving contact arm 16 is arranged to be rotated by a motor (to be described later) at a speed of 360 rpm. whereby the senders A to A are each operated six times per second.

The pecker bars of the fifteen senders are connected to the fifteen fixed contacts 1 to 15 respectively of a second distributor D whose moving contact arm 16 is earthed. The moving contact of the distributor D is .meehanically connectedyto the moving contact. of the distributor D by any suitable means shown as a broken line S whereby the two moving contacts rotate at the same speed. The pecker bars of the senders A to A are therefore earthed in turn, and each pecker bar is earthed six times a second. The moving contact of the ,;distributor D is arranged to lag relative to the distributo: D as shown.

Five diode 'rectifiers R to R are provided for each of the senders A to A and the peckers of each sender are connected to the anodes of the five rectifiers respectively associated therewith. The cathodes of the fifteen .sets of five rectifier-s R to R are connected to terminals T to T respectively. These terminals are connected through resistors P to P respectively to? the negative terminal of a 50 volt battery E whose positive terminal is earthed.

Theterminals T to T are normally, therefore, at a ipotential of -50 volts relatively to earth. Each time one or more of the peckers in the senders is earthed through its pecker bar and the distributor D however,

' the potential of the one of the terminals T to T to which the pecker is connected, is raised to earth potential. The pattern of the potentials of the terminals T to T is,

therefore, determined at any one instant by the code of the perforations in the tape in the sender whose pecker bar is earthed at that instant. This pattern is determined by the senders A to A in turn andchanges 90 times a second (six per second per sender) that is to say each pattern lasts for about 11.11 milliseconds.

It will be appreciated that the senders are each operated at the standard teleprinter rate or" six characters per second, and it is though that 15 channels is the most that can be operated satisfactorily in a time division multiplex system over long distance radio links.

It is well known that reflections by .the ionosphere of a long distance signal may cause theduration of a signal tovvary up to three milli-seconds- Signals of durations 7 less than about ten milliseconds are liable, therefore to be mutilatedto the extent of producing errors at a re Referring now to Figure 1(b) the electrostatic generaapertures AP formed therein. The wheel W is rotated by a synchronous motor M supplied with power at 60 c./s. from a tuning fork TF The tuning fork TF is arranged to be maintained in oscillation in known manner at a frequency of 1800 c./s. to within one part in a million. The tuning fork drive may be Type F .KLZ Fork Drive made by Times Telephoto Equipment Incorporated. The output from the fork is passed through a 30:1 frequency divider FD and a power amplifier PA .to the motor M A series motor SM is employed for running the wheel up to the synchronous speed of 1800 r.p.m., and the shaft of the motor SM is coupled through a gear GR of 5:1 ratio to a shaft S which may be directly connected to the moving contacts of the distributors D and D of Figure 1(a).

Thirty-two collector electrodes (not shown) are arranged close to the rings of apertures respectively in the wheel W, and the wheel is given a suitable potential by any suitable means (not shown). The number of perforations in the thirty-two rings is arranged to be such that the apertures in the thirty-two rings pass the collector electrodes respectively at the rate of 1800 per second, 1890 per second, and so on, in steps of to 4590 per second. Audio-frequency oscillations of frequencies 1800 c./s. and in 90 c./s. steps to 4590 c./s. are therefore made available at the collector electrodes. These electrodes are connected to the thirty-two cores of a thirty-two core cable CA. v

An additional ring of three, equally-spaced apertures AP is provided in the Wheel W for the generation of triggering pulses for a purpose to be described later. A collector electrode (not shown) co-operates with the additional ring of apertures AP and is connected to a terminal T Pulses therefore appear at the terminal T at the rate of one pulse every 11.11 milliseconds. An alternating current generator ACG is connected to the shaft of the wheel W and provides an output of 90 c./s. for a purpose to be described later. The output of this genorator is applied to an output terminal T Referring now to Figure 1(a) this shows a selector arrangement for selecting oscillations from the arrangement shown in Figure 1(b) in dependence upon the potentials on the terminals T to T of Figure 1(a). The cable CA and terminal T are those shown in Figure 1(1)), and terminals T to T are those shown in Figure 1(a). The terminals T to T are connected to the control terminals of five gates G to G respectively whose input terminals are connected together andto the output terminal of a Kipp relay K that is to say. a relay which provides anoutput pulse some time after the application thereto of anoperating pulse. The five gates G to G may conveniently be pentode valves, the terminals T to T being connected to the suppressor grids of the five pentodes respectively, and the control grids being connected together and to Kipp relay K The Kipp relay may be a form of one shot multivibrator such as is described in Principles of Radar. by the stalf of the Massachusetts Institute of Technology, published by McGraw Hill, Chapter 2, Article 15..

The outputs of the gates G to G are applied to one of the input terminals of five flip-flop circuits F to F respectively a flip-flop circuit being a circuit having two input terminals and which can assume either of two stable conditions on the application of apulse to one ofthe other of the two input terminals. A common form of flip-flop circuit is one in which two triode valves are provided, the anode of each being connected to the con .is conducting and the, otheris non-conducting, and the.

other stable condition is that in which the valve'originally l conducting is non-conducting and. that originally, nonconducting is conducting. The application of a positivegoing pulse to the control grid of the valve which is nonconducting causes this valve to become conducting and the other to become non-conducting. Similarly the application of a negative-going pulse to the control grid of the valve which is conducting causes this valve to become non-conducting and the other to become conducting. The two input terminals of the flip-flop circuit may therefore be connected to the control grids of the two valves respectively, and two output terminals may be connected to the anodes of the two valves respectively.

In Figure 1(a) the outputs of the gates G to G are connected to one of the input terminals of the five flipfiop circuits F to F as already described, and the other input terminals of the flip-flop circuits F to F are connected to the output terminals of five bufi'ers B to B respectively. The input terminals of the buffers B to B are connected together and to one output terminal of a second Kipp relay K which may be of the same type as K A second output terminal of the Kipp relay K is connected to the input terminal of the Kipp relay K The delay of the Kipp relay K can be varied by a variable delay device VAD, and the pulses appearing at the terminal T every 11.11 milliseconds are applied to the input of the Kipp relay K The action of the arrangement of Figure 1(c) as so far described may be as follows:

A pulse applied to the Kipp relay K produces a sharpened pulse at the output of the relay K after a short delay of a few microseconds. The output pulse is applie to the inputs of the five butters B to B and passes thence to the five flip-flop circuits F to F This sets all five flip-flop circuits in the same condition. The output pulse from the Kipp relay K is also applied to the Kipp relay K which is arranged to provide an output pulse about ten microseconds later. This pulse is applied to all five gates G to G and is reproduced in the outputs of the gates whose control terminals are connected to those of terminals T to T which are at earth potential. The remaining gates provide zero output. Those of the five flipfiop circuits F to F connected to the gates which pass the applied pulse are caused to assume their second stable condition.

The five flip-flop circuits are, therefore, set up in conditions determined by the code of the set of perforations in the tape engaged by the packers of the one of the senders A to A (Figure 1(a)), Whose pecker bar is earthed at that instant through the distributor D The flip-flop circuits F to F remain in those conditions until reset by the next pulse applied to the Kipp relay K from the terminal T Out of every 11.11 milliseconds (the time between successive pulses applied to the terminal T and the time between successive movements of the tapes in the fifteen senders A to A of (Figure 1(a)) a period of only 10 microseconds is used to set up the flip-flop circuits F to F This allows considerable latitude in the resetting (notching-on) of the tapes in the senders A to A (Figure 1(a)), in the movement of the peckers during the notching-on periods, and in the positions of the moving contacts 16 in the distributors D and D The two output terminals of the five flip-flop circuits are connected to the input terminals of five push-pull bulfer stages B to B respectively, and the outputs of the five buffers B to 13 are applied to the five pairs of input terminals of a matrix GM of germanium rectifiers'. A suitable rectifier matrix is described in Proc. I.R.E., February 1949, page 139, under the title Rectifier Networks for Multiposition Switching, by D. R. Brown. The matrix has thirty-two output terminals and it is arranged in known manner that irrespective of the outputs of the five bufier stages 13,; to B as determined by the settings of the five flip-flop circuits F to F only one of the output terminals is at any instantat earth potential, the remainder being at say 10' volts negative relatively to earth. The thirty-two different combinations of the setting of the flip-flop circuits F to F cause the potential of different ones of the thirty-two output terminals of the matrix to be raised to earth potential.

The thirty-two output terminals of the matrix GM are connected to the control terminals of thirty-two gates respectively, shown as a block G Only one of these gates is open, therefore, at any one instant. The cores of the thirty-two core cable CA are connected to the input terminals of the thirty-two gates G respectively and it is arranged that the thirty-two gates have a common output terminal T The frequency of the oscillation at the terminal T at any instant is determined by the code of the perforations in the tape in that one of the senders A to A which has its pecker bar earthed at that instant through the distributor D of Figure 1(a). The frequency of the oscillations at the terminal T changes abruptly at the rate of times per second when different characters are successively transmitted. It will be remembered that the thirty-two oscillations generated by the arrangement of Figure 1(c) are all harmonically related. The purpose of arranging the frequency relationships of the oscillations in this way is to facilitate the suppression of transients when the frequency of the oscillations at the terminal T makes an abrupt change. By suitable selection of the instants of occurrence of the pulses at the terminal T or by suitable adjustment of the variable delay device VAD associated with the Kipp relay K it can be arranged that the abrupt changes in frequency at the terminal T occur when the oscillations are passing through zero.

Referring to Figure 1(d) this shows a limiter L followed by four flip-flop circuits F to F connected in cascade. The flip-flop circuits F to F are arranged in known manner to act as frequency dividers each having a division ratio of 2:1. Suitable frequency dividers are described in Principles of Radar by the staff of the Massachusetts Institute of Technology, published by McGraw-Hill, Chapter 2, Article 14. An output is taken from each of the flip-flop circuits F to F and applied to the four fixed contacts respectively of a selector switch SW The moving contact of the selector switch SW is connected to a modulator MD to which is also fed the 90 c./s. oscillation at the terminal T The oscillations applied to the modulator from the switch SW are amplitude rnodulated therefore at 90 c./s. and it is arranged that the depth of modulation is about 50%. The purpose in providing this modulation is to facilitate synchronisation at a receiver as will be described late. The output of the modulator is passed through a cathodefollower stage CH and a transformer N to a low-pass filter Z having a cut-off frequency of 5000 c./s. The output of the low-pass filter is applied to a transmitter TR by means of a suitable transmission line. At the transmitter the 90 c./s. modulation is removed and the demodulated tones are applied to produce frequency shift of a carrier in known manner. The frequency of the frequency-modulated carrier is then multiplied and the 90 c./s. modulation reapplied. The switch SW is given a setting dependent upon the degree of frequency multiplication employed in the transmitter TR, whereby the modulation frequencies transmitted from the transmitter TR are the same as those appearing at the terminal T8.

It is arranged that a pilot carrier of low amplitude is also transmitted giving the effect of a single sideband with a pilot carrier. As only one frequency is transmitted at a time the whole of the transmitter power (less what is required to transmit the pilot carrier) is transmitted at that frequency. It is preferred to operate the transmitter under class C conditions.

Receiving equipment will now be described which demodulates signals received from the transmitter TR of Figure 1(d), measures the frequency of each 11.11 millidiodes into a ditferentiating network.

I the timing pulses.

seconds burst of oscillations received to decide which it is of .the thirty-two :possible frequencies, establishes a five-unit code corresponding to that at the transmitting terminal causing oscillations of the identified frequency :to be transmitted, distributes the five-unit codes to fifteen channels, and produces voltages in each channel of the :form necessary for operating a standard teleprinter.

Each burst of oscillations lasts for 11.11 milliseconds, and as previously explained, the ionosphere may cause changes in the durationof each signal up to about 3 milliseconds. An arrangement will be described whereby the 'first and last quarters of each signal are disregarded and measurements are made on the centre portion, of 5.55

90 c./ s. modulation is reapplied to produce an audio tone signal substantially identical with the modulation component at the output of the transmitter TR of Figure 1(d). This audio tone signal is passed from the radio receiving equipment (not shown) to the multiplex receiver where is is applied to the input terminals T and T of Figure 2(a). These terminals are connected through a switch SW and a transformer N to an input control circuit 1C which may be any suitable variation attenuator. The signals are applied from the input control circuit 1C to amplifier AMP and thence to twophase-changing circuits PC and PC These circuits rovide two outputs whose phases differ by 90 over a wide frequency range, for instance from 300 to 6000 c.p.s. Examples are given in an article by R. B. Dome .in Electronics, December1946, page 112. The phase shift effected by the circuit PC; is made 45 and that effected by the circuit P02 is made -45 irrespective of frequency. The outputs of the two-phase-changingcir- 'cuits PC and PC are applied to a pulse-forming circuit oscillation and may operate by limiting to a square wave form'in a push-pull circuit and feeding the push-pull outputs from the anodes of the push-pull circuit through in this way one positive (or negative) pulse is obtained foreach crossing of the means potential in the input wave-form. A further pulse forming circuit PE is provided for a purpose hereinafter to be described. For convenience the train of pulses at +45 will be referred to as the working pulses and the train of pulses at -45 will be referred to as The working pulses are applied through a cathode follower stage CF to a terminal T and the timing pulses are passed through a switch SW and a cathode follower stage C1 to a terminal T A connection is made from the secondary winding of the transformer N to a terminal T for a purpose to be described'later.

Referring to Figure 2(1)) a tuning fork TF is kept in oscillation in known manner at a frequency of 1800 c./s.

to within one part in a million. Electrical oscillations at 1800 c./s. derived from the tuning fort; are passed to a synchronisingcircuit SC to be described later. The synchronising circuit produces a. pulse from each half-cycle of the1800 c./s. oscillation and hence the output of the synchronising circuit is in the form of pulses having *8 example as already described) -to function -as frequency dividers each having 'a division ratio of 2: 1. The output of the flip-flop circuit E is :ther'efore in the form of pulses having a recurrence'frequency of 900 pulses :per second.

The output of the flip-flop circuit P is applied to a counter DR. Thisisin the form of what is commonly known as a decade ring and is of the kind describedby V. H. Regener in Review of Scientific Instruments, 1946, volume '17, page 185. The rotational cycle of the counter is repeated for every ten pulses applied and as the pulses have a frequency of 900 pulses per second there are 90 complete cycles every second, or one every 11.11 milliseconds.

As output is taken from No. 1 flip-flop circuit in the counter DR through a buffer stage B to aterrninal T14,'8I1 output is taken from No. 7 flip-flop circuit through a buffer stage B to an output terminal T an output is taken from'No. 8 flip-flop circuit through a buffer stage 3 to an output terminal T and an output is taken from No. 9 flip-flop circuit through a buffer stage B to an output terminal T17 for purposes to'be described later.

'It is arranged that each time the flip-flop circuits Nos. 1,

7, 8 and 9 are turned off they transmit a pulse to their respective output terminals T to T The outputs of flip-flop circuits Nos. 1-, 3, 5 andB'areconnected through buifer stages B to 13 respectively to an output terminal T Each output delivers a c./s. pulse train and when combined produces a 360 c./s. pulse train at terminal T for a purpose to be described later.

Referring to Figure 2(a) the terminal T is that shown in Figure 2(b) and is connected to one input terminal of a flip-flop circuit F The terminal T is that shown in Figure 2(a) to which the timing pulses are applied, and is connected to the input terminal of a gate G The output terminal of the gate G is connected to the other input terminal of the flip-flop circuit F Each time a pulse appears at the terminal T 4, that is to say every 11.11 milliseconds, the flip-flop circuit'F 12 is "switched on. Whenever the circuit P is switched on it applies a voltage to the control terminal of the gate G which then opens. The next timing pulse appearing at the terminal T after the gate G is opened passes to the flip-flop circuit F and switches it off. Each time the circuit P is switched off output voltage therefrom serves to close the gate G and to switch on a further flip-ilop circuit F This flip-flop circuit when switched on applies voltage through a cathode follower C15 to an output terminal T and to the input terminal T of a timing device CLC to be described later. The timing device "CLC is arranged to provide a pulse at its output terminal T 5.5 3' milliseconds after the pulse applied to the input terminal T The output pulse at the terminal T is "applied 'to the flip-flop circuit F to switch it oif. As the pulses applied to the terminal T occur every 11.11 milliseconds, the voltage at the terminal T is in the form of an oscillation of square waveform, each half-cycle having a duration of 5.55 milliseconds and commencing at the instant of occurrence of a timing pulse: applied to the terminal T It will beremembered that the timing pulses lag 90 on the working pulses applied to the terminal T of Figure 5 and hence each half-cycle of the oscillation of square wave form at the terminal T com.- mences midway between two working pulses.

Figure 2((1) shows in more detail the arranger'n'entof the timing device CLC of Figure 2(c).' A crystal-controlled oscillator CCO of a frequency of 92.16 kc./s. is

connected to a flip-flop circuit'F which is. arranged to derivepulses at a recurrence frequency of' 92160 pulses.

per secondfrom the oscillations of 92.16 kc./s. These pulses are applied to agate G whose control terminal is the terminal T shown in Figure 2(a). Each time the gate G is opened by voltage applied to the terminal T the pulses applied to the gate' from the flip-flop circuit 11 being transmitted; These five signals are followed by a stop signal lasting about 33 milliseconds during which current of fixed magnitude is passed into the teleprinter. An example of such a series is shown in Figure 3 by a curve TPS. In Figure 3 the ordinate represents magnitude, the abscissa represents time, the intervals t to i are each of about 22 inillisecoiids and the interval is of 33 milliseconds. The signal commences at the beginning of the interval 1 and ends at the end of the interval 2 It will be seen that during the interval 1 (start signal) and intervals t and it zero current is flowing, so that during the intervals 1 t t and t a current of amplitude h is flowing. The signals t to i carry the information which determines the character to be printed by the receiving teleprinter. Those of signals to t of zero amplitude are usually termed space signals and those of amplitude it are usually termed mark signals. Including the combination having five space signals and that including five mark signals there are 32 possible combinations of mark and space signals.

Referring again to Figure 2(e) it is arranged that when any of the flip-flop circuits F to P are switched on and their associated gates G to G are open, their associated terminals T 2 to T are at earth potential. it is also arranged that when the gates G to G are closed, or when any of the flip-flop circuits F to P are switched off and their associated gates are open, their associated terminals T to T are at negative potential. Whenever the gates G to G are opened the respective output terminals T to T assume either earth potential or a positive potential dependent upon whether the respective flip-flop circuits F to F are switched on or off. Including the condition in which all the terminals T to T are at earth potential and that in which all the terminals T to T are at the positive potential, there are thirty-two possible combinations or the potentials at the terminals T to T These combinations are determined by the received audio oscillations as previously described, and the distributor about to be described serves to convert these combinations of potentials on the terminals T to T to signals having the pre- 'viously described form (of which one example is shown in Figure 3), as well as to distribute the successive combinations of potentials appearing at the terminals T to T to channels.

A part of the distributor is shown in Figure 2(1) and comprises a ring of conducting segments or contacts C to C which are insulated from one another. A slip ring SL is disposed on one side of the segments C to C and a second slip ring SL is disposed on the opposite side. The two slip rings SL and SL are arranged (in any suitable manner) to have potentials +100 volts and 1G0 volts rsepectively relatively to earth. Batteries E and E are shown for this purpose. Two brushes BR and BR are arranged to make "connections between the slip ring SL and the contacts C to C in turn as the brushes are rotated. A third brush BR is arranged to make a connection between the slipring SL and the contacts C to C as this brush is rotated. The three brushes BR BR and BR are arranged to be ro tatcd in step by a shaft SH in the direction of the arrow whereby the brush BR lags on the brushes BR and 3R The brushes BR and BR' are arranged to be in contact with two adjacent. contacts at any instant, these brushes being shown in contact with contacts C and C respec 'tively in the drawing. At the samejinstant the. brush BR is made to be in contact with the contact next preceding that in connection with the brush BR In the drawing the brush BR is shown to be in connection with the contact C The shaft SH is coupled to a 60 c./'s. synchronous motor M by any suitable gearing of 5:1 ratio shown in the drawing by a broken line 8H The shaitot the motor M is arranged to rotate at 1,800 -r-.p.'rn. whereby the'brush'es BR1,BR2 and BR, make six complete revolutions every second, that is to say, aproxirnately one every 166 milliseconds. In this way each of the brushes remains in contact with each of the contacts C to C in turn for a period of 11.11 milliseconds.

Alternating current for the motor M is derived from the voltage appearing at the terminal T which is that shown in Figure 2(1)). This voltage is in the form of pulses occurring at a frequency of 360 pulses per second as hereinbefore explained. This pulse train is applied, as shown in Figure 2(1) to a six-stage ring counter CR of the kind described by V.H. Regener in Review of Scientific Instruments, 1946, volume 17, page 185. Opposed phase 60 c./ s. square wave outputs are taken from stages 2 and 5 of the counter CR and are applied to the inputs espectively of a balanced push-pull amplifier PPA whose output is applied to the motor M The motor M is therefore synchronised to the motor M of Figure 1(b).

For this purpose of further description it is preferred to represent the contacts C to C of Figure 2(f) in extended fashion as shown in Figure 2( g).

Figure 2(g) shows valve apparatus for supplying suitable voltages to a teleprinter in No. 1 channel. Fifteen sets of valve apparatus as shown in Figure 2(g) are provided, the apparatus for channel No. 2 being shown in Figure 2(h) to be described later.

Referring to Figure 2(g) seven thyratrons GT to GT are provided. These are of the type having two control grids as shown. In order to strike thyratrons of this type it is necessary to apply positive potential to the anode and both grids. A triode V is also provided for triggering the thyratrons GT to GT The cathode of the valve V is connected to earth, the control grid through a resistor P to a terminal T and through a resistor P to contact C of the distributor. The terminal T is arranged to have a potential of volts and the anode of the valve V is connected to this terminal through a resistor P The inner grids of the thyratrons GT to GT-; are connected through resistors P to P respectively and the common terminal of all these resistors is connected through a resistor P to the junction of two resistors P and P connected between a terminal T and earth. The terminal T is arranged to have a potential of 75 volts whereby the inner grids of the thyratrons GT to GT are normally negatively biased. The anode of the valve V is coupled through a capacitor Cl, to the common terminal of the resistors P to P The outer grids of the thyratrons GT 1 to GT are connected through resistors P to P tothe five terminals T to T which are those shown in Figure 2(e). The outer grids of the thyratrons GT and GT are connected through resistors P and P respectively to a terminal T which is arranged to have a positive potential whereby the outer grids of the two thyratrons GT and GT are normally positive. The cathodes of the thyratrons are connected together and through one winding of a differential relay REL, to earth. The anodes of the seven thyratrons GT and GT are connected through resistors P to P respectively, and the common terminal of these resistors is connected through the other winding of the diiferential relay REL to the terminal T The anode of the thyratrons GT to GT; are also connected through resistors P to P and rectifiers R to R respectively to contacts C C C C C C and C respectively of the distributor. The relay REL has a fixed contact FC connected to the terminal T at +100 volts, and a second fixed contact FC connected to earth. The moving contact MC; of the relay REL is connected to an output terminal T for connection to a teleprinter input terminal.

p In operation, as the brush 3R passes over the contact C the brushes BR and BR pass over contacts C and C -respectively. During this interval of 11.11 milliseconds a negative-going pulse of 100 volts is applied from the contact C to the control grid ofthe triode V causing the anode current of this valve to be cut off and a large posi= F pass through the gate circuits F to F each arranged as a frequency-divider ample is given in Electronics,

having a division ratio of 2:1. The overall division ratio of the chain is, therefore, 512:1 whereby the output of the last divider F in the chain is at a frequency of 180 c./s. and has a period'of 5.55 milliseconds. The output of the last divider F is passed through a pulse forming circuit PF to the terminal T which is that shown in Figure 2(a). The first pulse appearing at the terminal T occurs at an instant 555 milliseconds after the gate G is opened, to within one part in 92160. In this way the time interval of 5.55 milliseconds is measured with a degree of accuracy suflicient for the purposes of the present embodiment.

Referring to Figure 2(e), a gate G has its inputterminal connected to the terminal T which is that shown in Figure 2(a) and hence the working pulses are applied to the input of the gate G The control terminal of the gate G is connected to the terminal T which is that shown in Figure 2(0). The oscillation of square wave form applied to the terminal T serves, therefore, to open and close the gate G alternately for periods of 5.55 milliseconds respectively. It will be remembered that the bursts of working pulses are at any of the thirtytwo possible frequencies ranging from 3600 pulses per second in steps of 180 pulses per second to 9180 pulses per second. Irrespective, therefore, of the frequency of the pulses applied to the gate G during each interval of 5.55 milliseconds when the gate. G is open, a whole number of pulses are applied to the gate from the terminal T It will also be remembered that the timing pulses applied to the gate G (Figure 2(c)) are delayed 90 on the working pulses. Each cycle of the square wave oscillation applied to the terminal T commences, therefore, between two working pulses. The gate G always opens, therefore, between two working pulses and closes between two working pulses provided the pulses are not affected by noise. In this way possible errors in operation due to the gate (3,; opening and closing during working pulses, are avoided.

The synchronising circuit SC of Figure 2(b) serves, in a manner to be described later, to ensure that the gate 6,, opens about 2.77 milliseconds after the commencement of each burst of received oscillations applied to the terminals T and T (Figure 2(a)), whereby the first and last quarter of each received signal are prevented from passing through the gate G and only the centre portion of 5.55 milliseconds passes through the gate G During each 5.55 milliseconds when the gate G is open, working pulses pass through the gate G into a five-digit binary counter having five flip-flop circuits F to F arranged in known manner. These are two-state flipflop circuits arranged in cascade so that one of the circuits is operated from a preceding circuit. An ex- September '1948,'on page 111. At the beginning of each count the five flip-flop circuits F to F are in the switched-01f condition as will be described later. It is arranged that the first pulse applied to the counter switches on the flip-flop circuit F the next pulse switches off F and switches on F the third pulse leaves F switched on and switches on F the fourth pulse switches off F and F and switches on F and so on. In this way when the five flip-flop circuits are switched on they indicate counts of 1, 2, 4, 8 and 16 respectively.

The pulses applied to the binary counter maybe at any of the thirty-two frequencies ranging from 3600 pulses per second to 9180 pulses per second. The number of pulses passed into the binary counter during each operative period of 5.55 milliseconds may, therefore, be any of a series commencing at 20 and increasing in unit steps to 51. The application of that one of the series containing 32, leaves the flip-flop circuits F to F all switched off but the remainderleavesone or more of the flip-flop circuitsswitched on,- ditferent ones, or different combinations, of the flip-flop circuits beingleft switched on after the application of groups of different numbers of pulses in the series (excluding 32). It is arranged that the conditions of the'five flip-flop circuits F to F after the application of a burst of pulses thereto corre-- sponds to the conditions of the five flip-flop circuits F to F respectively of Figure 1, which determine the frequency of the pulses in the burst, that is to say, when flip-flop circuit F is switched on, flip-flop circuit F is switched on and so on.

The conditions of the five flip-flop circuits F to F correspond in a particular manner to the thirty-two oscillations which they cause to transmit. If F is switched off and the other four are switched on, an oscillation of frequency 2970 c./s. is transmitted, if F is switched off and the other four switched on, an oscillation of frequency 3060 c./s. is transmitted, and so on through the various codes in steps of 90 c./s. up to 4590 c./s., the highest frequency transmitted. The next code F F F on and F F 01f causes 1800 c./s. to be transmitted, and so on through the codes, in steps of 90 c./s. until all five flip-flops are switched off when an oscillation of frequency 2880 c./s. is transmitted In this way the oscillaeion which is transmitted can be converted to a five-unit code at the receiver directly without the need for filters.

It Will be remembered that the commencement of each 5.55 milliseconds period during which the gate G is open is determined by the appearance of a pulse at the terminal T (Figure 2(b)) connected to an output terminal of the No. 1 flip-flop circuit in the decade ring counter DR. About 1.11 milliseconds after the end of each 5.55 milliseconds period when the gate 6,, is open a pulse appears at the terminal T which is connected to an output terminal of the No. 7 flip-flop circuit in the decade ring counter DR (Figure 2(b) The pulse is applied to switch on a flip-flop circuit F (Figure 2(e)) whose output is applied through a cathode-follower CF to a common control terminal of'five gates G to G The outputs of the five flip-flop circuits F to F in the binary counter are applied to the input terminals of the five gates G to G respectively, and the outputs of the gates G to G are applied through five cathode-followers CF to CF respectively to five output terminals T to T Each time the gates G to G are opened by the flip-flop circuit F the terminals T to T assume potentials dependent upon the conditions of the five flip-flop circuits F to F respectively in the binary counter. V

About 1.11 milliseconds after the gates G to G are opened a pulse appears at terminal T This pulse is applied to switch otf the flip-flop circuit F which in turn closes the gates G to G and switches on a flip-flop This flip-flop circuit then applies a resettingvoltage to the five flip-flop circuits F to P 9 which are all switched 01? thereby. About 1.11 milliseconds later circuit F a pulse appears at the terminals T and is applied to switch off the flip-flop circuit F whereby the binary counter is left in a condition ready to start the next count. About 2.22 milliseconds later a pulse appears at the terminal T (Figures 2(b) and (0)) and the last described cycle of operations starts again.

An arrangement will now be described for distributing the voltages appearing at the terminals T to T to15 channels and for converting these voltages in each channel into a series of pulses of the standard form for operating a teleprinter. In standard teleprinter practice each transmitted character is represented by a series of signals, the series being of 166 milliseconds duration. Each series consists of a start signal lasting about 22 milliseconds during which no current is passed into the receiving teleprinter. This signal is followed by five further signals each lastingabout 22 milliseconds during which a current of fixed magnitude, or zero current, is passed into the teleprinter dependent upon the pharacter tive-going pulse to be applied to the inner grids of the seven thyratrons GT to GT The thyratrons GT and GT have positive potential on their outer grids from the terminal T and positive potential on their anodes through the resistors P and P respectively. These two thyratrons strike. It is arranged that the gates G to G (Figure 2(2)) are open for about 1.11 milliseconds and it will be remembered that the terminals T to T have potentials determined by a signal received in channel No. 1. The anodes of the thyratrons GT to GT have positive potential applied thereto through the resistors P to P respectively. Those of the thyratrons GT to GT whose outer grids are connected to those of terminals T to T which are at positive potential, strike, and the remainder stay non-conducting. The currents fiowing in the two windings of the relay REL are equal and hence, throughout this interval of 11.11 milliseconds, the moving contact MC is in contact with the earthed fixed contact FC and hence a space signal is sent to the terminal T The brushes move on and during the next five successive intervals, each of about 22 milliseconds duration, the contacts C C C C C are made 100 volts positive in turn by the brushes BR and BR These five contacts are connected through the rectifiers R to R respectively to the anodes of the five thyratrons GT to GT A positive-going pulse of 22 milliseconds duration is, therefore, applied to these anodes in turn. The cathode current of each of the thyratrons GT to GT which is struck is therefore increased whereby the relay REL is operated causing a mark signal to be transmitted to the terminal T The cathode current of each of the thyratrons GT to GT which is non-conducting remains unaltered whilst the positive pulse is applied to the anode thereof, and hence a space signal is sent to the terminal T In this way five mark and/or space signals each of about 22 milliseconds duration follow each other in succession in dependence upon the potentials applied to the terminals T to T during the interval when the brush BR was on the contact C The brushes move on and BR and BR in passing over contacts C and C in succession cause a positive-going pulse of about 22 milliseconds duration to be applied to the anode of the thyraton GT and a positive-going pulse of about 11.11 milliseconds duration to be applied to the anode of the thyraton GT The sum of the durations of the increased cathode currents of GT and GT, is, therefore, increased and the relay REL is operated for a periodof about 33 milliseconds, and hence a stop signal of that duration is applied to the terminal T Each of the positive-going pulses applied to the anodes of the thyratrons GT to GT is followed by .a negativegoing pulse from the brush BR which extinguishes those of the thyratrons GT to GT which were struck. The brush -BR on reaching the contact C causes the thyratron GT to be extinguished and on reaching contactC causes the thyratron GT to be extinguished, and the cycle of operations is completed. During the final 11.11 milliseconds interval of the cycle no current flows in the relay REL; and hence a space signal is applied to the terminal I This space signal in combination with the space sig- -nal occurringlduring thefirst interval of 11.11 milliseconds of the cycle of operations described constitute a teleprinter start signal. H X

' Itfwill be seen, therefore, that over each interval of 166 milliseconds (one complete revolution of the brushes) 'there is transmitted to the terminal T a start signal of 22 milliseconds duration followed by a combination of five mark and/orspacesignals in succession each of about 22 milliseconds duration, followed by a stop signal of about 33 milliseconds duration, the combination of mark and/or space signals representing a character to be printed by 'thechannel No. 1 teleprinter and determined by the automatic sender A in Figure l. The cycle of operations-.is repeatedat the rate of six cycles per second whereby the teleprinter in channel No. 1 prints the char- "14 acters as received in channel No. 1, at the standard rate of six every second.

The apparatus for channels Nos. 2 to 15 is the same as that shown in Figure 2(g) with the exception that the connections are such that each cycle of operations in the successive channels commences 11.11 milliseconds after that in the preceding channel. For example, the cycle of operations in the apparatus for channel No. 2 commences 11.11 milliseconds after that described with reference to Figure 2(g). This is achieved by making the connections between the valves V and the thyratrons GT and the contacts C to C in a suitable manner. For example, referring to Figure 2(h) this shows the apparatus for channel No. 2. The function of the valve V, is the same as that of the valve V in Figure 2(g). Similarly the functions of the thyratrons GT to GT are the same as those of the thyratrons GT to GT respectively in Figure 2(g). In the arrangement of Figure 2(h) it will be seen that the anodes of the thyratrons GT to GT are connected to contacts C C C C and GT instead of to contacts C C C C and C as is the case in Figure 2(g). The control grid of the valve V is connected to contact C instead of to contact C as is the case in Figure 2(g). In this way each cycle of operation of the arrangement shown in Figure 2(h) is made to commence 11.11 milliseconds after that of the arrangement of Figure 2(g).

Referring now to Figure 2(i), this shows in more detail the synchronising circuit SC of Figure 2(b).

In Figures 2(1') the tuning fork TF and the flip-flop circuits F and F are those shown in Figure 2(b), the terminal T is that shown in Figure 2(a). The terminal T is, of course, that also shown in Figure 2(0). The Wave forms of voltages present in various parts of the circuit are shown adjacent those parts in the drawing. The c./s. modulations of the oscillations appearing at the terminal T is extracted by means of a demodulator DM and passed through a pulse forming circuit PP of known kind to produce one positive-going pulse per cycle of the 90 .c./s. oscillation. These positive-going pulses are applied through a switch SW to a Kipp relay K whose delay is made adjustable in known manner to a maximum value of about three milliseconds. The output pulses of the Kipp relay K are applied to the input terminals of two gates G and G The Voltage of square Wave form appearing at the terminal T is applied through a phase-splitter PC which provides two output voltages in anti-phase which are applied to the control terminals of the gates G and G respectively. These two gates are opened alternately, therefore, for periods of 5.55 milliseconds each. Each pulse applied to the input terminals of the gates G and G from the Kipp relay K passes through one or the other of the gates G and G (depending upon the relative phases of the pulses from the Kipp relay K and the two outputs respectively of the phase-splitter PC but never passes through both.

The output of the tuning fork TF is passed into two phase-shifters PC and PC which produce phase-shifts of +45 and 4S respectively. The oscillations at 1,800 c./s. are then passed from the two phase-shifters PC; and PC toflip-fiop' circuits F and F respectively acting in known manner to produce one pulse for each half-cycle of the oscillation applied thereto. The outputs of the flip-flop circuits F and F consist, therefore, of two trains of pulses at 3,600 pulses per second displaced 90 relatively to one another.

The pulses from the fiip-flop circuit F are applied to the input terminal of a gate G The pulses from the flip-lop circuit F are applied through a buffer stage B to one input terminal of a flip-flop circuit F and serve to switch this flip-flop circuit on. When the flip-flop circuit P is switched on a voltage is applied therefrom to the control terminal of the gate G to open this gate. Pulses are passed, therefore, from the flip-flop circuit'F through the gate G which is normally open, to the flip- F is switched off a voltage is applied therefrom to close the gate G18. I

The other input terminals of the flip-flop circuits F and F are connected, however, to the output terminals of the gates G and G respectively. Each time a pulse appears at the output of the Kipp relay K either the flip-flop circuit F is switched off, or the flip-flop circuit F is switched on thereby, depending whether the gate G or G is open at the instant when the pulse appears at the output of the Kipp relay K If the gate G is open, the pulse passed thereby serves to close the gate G to one pulsefrom the flip-flop circuit F On the other hand, if the gate G is open then the gate G is opened to pass one pulse from the flip-flop circuit F The number of pulses applied to the flip-flop circuit F in each period of 11.11 milliseconds is, therefore, either 39 or 41 but never 40, which is the number of pulses supplied in 11.11 milliseconds by each of the flipfiop circuits F and F The frequency of the pulses applied to the flip-flop circuit F determines the rate at.

which the counter DR of Figure 2(b) completes each cycle of ten operations and this in turn determines the instant at which each half-cycle of the oscillation appearing at the terminalT commences. I

It will be assumed that at a given instant a positive half-cycle of the oscillation appearing at the terminal T is just commencing and'a burst of oscillations is just commencing at the terminals T and T of Figure 2(a). It will be remembered that each positive half-cycle of the oscillation appearing at the terminal T is required to start about 2.8 milliseconds after the commencement of a burst of oscillations at the terminals T and T of Figures 2(a), whereby the gate G, of Figure 2(e) opens for 5.55 milliseconds over only the middle portion of a received signal to facilitate accurate counting by the flipflop circuits F to F The delay of the Kipp relay K is set to provide'an output pulse 2.8 milliseconds after the arrival of each burst of oscillations at' the terminals T and T It is arranged that positive half-cycles of the oscillation appearing at the terminal T open the gate G and that negative half-cycles open the gate G The pulse appearing at the output of" the Kipp relay passes, therefore, in this example, through the gate G causing the counter DR of Figure 2(b) to be slowed down whereby the phase of the oscillation at the terminal T is retarded. This is repeated several times until the commencement of each positive half-cycle of the oscillation at the terminal T is retarded; This is repeated several times until the commencementof each positive half-cycle of the oscillation at the terminal T substantially coincides with the instants of occurrence of the pulses at the output of the Kipp relay K The system thenhunts about, andclose to, this condition, one or i a perhaps two pulses passing through gate- G to slow up the counter DR (Figure 2(b)) and then one or perhaps two pulses passing through the gateG to speed up the counterDR. Thereafter, irrespective of' the fact that V the instants of commencement of the bursts of oscilla? tions at the terminals T and T may be delayed or ad vanced owing to ionospheric effects orgcauses, the synchronising circuit ensures that these changes are followed as they occur whereby the centre portion of 5.55jmilliseconds of eachfsignal is the only portion which is used; Furthermore, as the synchronous motor M in the distributor arrangement describedwith reference to' Figures 2(1)), (g) and.(h) is supplied-with power from the '16 counter DR of Figure 2(b) the distributor also follows these changes.

No special provision is made'for continuously controlling the synchronism of the 15 channels, that is to say, to insure that signals received in channel No. 1 go to teleprinter No. l and so on. With such rigid control over phase as previously described, channel synchronism is automatically maintained under normal conditions. It is necessary merely to arrange that the channels are correctly set up in the first instance.

In order to set up the 15 channels it is arranged that initially in 14 of the channels the same signal is sent repeatedly, say bursts of oscillations at 4,590 c./s., and in the remaining channel, say No. l, a signalof, say, 1,800 c./s., is transmitted repeatedly. This may conveniently be arranged by a simple switching circuit.

At the receiving end the receiver is disconnected from the terminals T and T (Figure 2(a)) by means of the switch SW and a tone at 1,800 c./s. is switched to the transformer N The output of the receiver is, however, arranged to be passed from terminals T T to a resonant circuit tuned to 1,800 c./s. The output of the resonant circuit is applied to the vertical deflecting plates of a cathode ray tube and a time base circuit is connected to the horizontal deflecting plates of the tube. The time base circuit is arranged to produce linear deflection of the beam at a frequency of, say, 6 c./s.

. A pulse is derived from the distributor in any convenient manner each time the brush BR makes contact with contact C and this pulse is also applied to the vertical deflecting plates of the cathode ray tube. Two vertical pulses appear, therefore, in the trace onthe screen of were less than about 10' decibels. By making the spacing the tube. .When these two pulses coincide channel synchronism is achieved. In order to bring this about the switch SW (Figure 2(i)) is set in the opposite condition to that shown, and one of the outputs of the phase-splitter PC is passed, through a differentiating circuit CP P to produce short pulses. These pulses pass through the switch SW through the Kipp relay K to the gates G15 and G15. I I The distributor is, therefore, slowed down and the switch SW4 is held in this condition until the vertical pulses on, the screen of the cathode ray tube coincide. The switch SW (which may convenientlybe'spring loaded) is returned to the condition shown in the drawing. Normal working in the system can then be started.

As the transmitter and receiver employ precision tuning forks, TF and TF respectively, synchronism of the transmitter and receiver is maintained for a period up of the transmitted oscillations say 180 c./s. instead of 90 ;c./s., the minimum acceptable signal-to-noise ratio would,

be about 7 decibels. Onthe other hand by making't-he 1 spacing 45 1c./s. the minimum acceptable signal-to-noise ratio would be about 13 decibels.

" r it will be seen, therefore, that for any given minimum 'signal-to-noise .ratio' a system of the appropriate band to-noise ratio is likely to be 40 decibels or more, the

channel spacing need be no more than'about 2 or 3 *c./s'."

.both outputs of the frequency deviation being employed.

Facilities may be provided whereby the receiver can be switched for use with different channel spacings. In the arrangement described, for example, the moving contacts of the switch SW (Figure 2(a)) may be connected to the lower pair of fixed contacts of the switch whereby pulse-forming circuit PF are applied to the terminal T In addition a further pulseforming circuit PF is brought into operation which supplies pulses to the terminal T12 and it is arranged that the latter pulses occur between those appearing at the terminal T In this way operation with 45 c./s. channel spacing may be effected.

In the foregoing description with reference to Figure 1(d), it is said that a pilot carrier of low amplitude is transmitted with the single sideband signals. The purpose of transmitting the pilot carrier is of course to facilitate demodulation at the receiver.

It will be understood that in order to ensure accurate reproduction of the transmitted characters at the receiver, the oscillations heterodyned with the single sideband signals must be extremely stable. This arises from the fact that the binary counter in Figure 2(a) converts the pulses applied thereto in each operative period of 5.55 milliseconds into a five-unit code for operating a teleprinter. If, therefore, the frequency of the oscillations heterodyned with the single sideband signals varies, the number of pulses applied to the binary counter in each operative period of 5.55 milliseconds varies accordingly with the result that the wrong five-unit code is set up.

The pilot carrier may be amplified and limited and then used as the local oscillation in the receiver. Owing to ionospheric effects giving rise to fading, however, the pilot may at times become masked by noise, resulting in unintelligibility in the reproduced characters. In order to overcome this difliculty to some extent, a local oscillator may be'employed in the receiver, andthe pilot carrier applied to control the frequency of the local oscillator in known manner. Provided the frequency of the local oscillator is made sufficiently stable to maintain its correct frequency to within about five parts in 20 million 'in the absence of the controlling pilot carrier due to fading, satisfactory results may be achieved.

It is also said in the description that the transmitted bursts of oscillation are modulated in amplitude at 90 cL/s. This is for the purpose of synchronising at the receiver as hereinbefore described. It will be appreciated, however, that the 90 c./s. modulation may be applied in addition, or alternatively to the pilot carrier. It may be conveniently arranged to apply the 90 c./s. modulating voltage to frequency-modulate the pilot carrier: a small The mean frequency of the pilot carrier may then be utilised for frequency control in the receiver and the 90 c./ s. frequencymodulation may be used, after demodulation, for synchronising as hereinbefore described. Such an arrangement facilitates receiver design because amplitude-modulation need not be used, and hence straightforward amplification and subsequent limiting may be employed whereby automatic gain control can be dispensed with.

I claim:

1. A transmitter for use in an electric signalling system, comprising means for producing different selections of pulses from a group of pulses, a plurality of sources of oscillations of different frequencies, a corresponding plurality of gate devices, means to apply said oscillations to said gate devices respectively, a common output terminal for said gate devices, and control apparatus responsive to different ones of said selections from the said group to open corresponding different ones of said gate devices.

2. A transmitter according to claim 1, wherein the control apparatus comprises a rectifier matrix having a number of input control circuits for said matrix equal to the totalnumber of pulses in the said group, a plurality of output terminals on said matrix and means connecting the said output terminals to the said gate devices respectively to control the opening and closing thereof, the input control circuits being responsive to the said selections to apply control voltages to the matrix, and the matrix being arranged in such a manner that only one of the gates is opened thereby at any instant irrespective of the voltages applied to the matrix from the input control circuits.

3. A transmitter according to claim 1, comprising a plurality of said sources each providing signals in the form of selections of pulses from a group, and a distributor to connect said sources in turn to said responsive means, whereby multichannel communication can be established.

4. A transmitter according to claim 3, wherein each of the said sources comprises an automatic sender for generating teleprinter signals, each teleprinter signal being generated by a selection from a group of five pulses, and wherein means are provided to synchronise said senders with said distributor to generate said teleprinter signals in turn.

5. A receiver for receiving successive bursts of oscillations of different frequencies representing different selections of pulses from a group of pulses comprising a demodulator for demodulating said received bursts of oscillations, and a converter for converting each demodulated burst of oscillations into a coded group of output pulses, the code being related to the selections made at the transmitter to generate the said burst.

6. A receiver according to claim 5, wherein the converter comprises a pulse former to produce a burst of working pulses in response to each burst of oscillations applied thereto and a counter to provide the coded groups of output pulses, in response to the burst of working pulses.

7. A receiver according to claim 6, comprising a primary gate, means to apply each burst of working pulses to said counter by way of said primary gate and a control circuit to open said primary gate only during a centre part of each burst of working oscillations applied thereto.

8. A receiver according to' claim 7, wherein the control circuit comprises a second gate, means for applying to the input terminal of the second gate timing pulses which occur between successive ones of the working pulses a synchronising circuit adapted toopen the second gate to allow a timing to pass therethrough at an instant after the commencement of a burst of working pulses, a clock device adapted to provide, in response to a timing pulse passed by the second gate, a gating pulse which, in operation, is applied to open the primary gate and has a duration such that the primary gate closes before the end of the last named burst of working pulses and between two working pulses.

9. An electric signalling system comprising means for selecting various combinations of pulses from a group of pulses, means responsive to each of the said combinations to generate a burst of oscillations having a frequency which is individual to each pulse combination, means to transmit said bursts, a receiver to receive said bursts and comprising, means to convert each burst of oscillations into a coded group of simultaneous pulses, means for synchronising said converter with the received bursts, and means for converting said coded group of simultaneous pulses into a corresponding code of successive pulses.

10. A transmitter for transmitting information in the form of electric signals, comprising means for presenting the information in a one-out-of-m pulse code, and transmitting means for transmitting in succession bursts of oscillations the frequencies of which bear a predetermined relationship to the numbers which successive coded pulses represent in said code.

11. A transmitter according to claim 10, wherein the frequencies of successivebursts of oscillations bear a 19 linear relationship to the numbers which successive coded pulses represent in said code.

12. A transmitter according to clairnlO, wherein the said transmitting means comprises meansto generate a plurality of oscillations of predetermined different frequencies, a corresponding plurality of gate devices, means to apply said oscillations to said gate devices respectively, a common output terminal for said gate devices, and means to apply different ones of said eoded'pulses to open corresponding diiierent ones of said gatedevice.

13. A transmitter according to claim 12, comprising means for converting information in the form-of different selections of pulses from a group of pulses to pulses in said one-out-of-m pulse code.

14. A transmitter according to claim 13, wherein said means for converting information comprises a rectifier matrix having'a number of input control-circuits for said matrix equal to the total number of pulses in the said group, a plurality m of output terminals on said matrix and means connecting the said output'terminals to the said gate devices respectively to control the opening and closing thereof, the input control circuits being responsive to the said selections to apply control voltages to the matrix, and the matrix being arranged in such a manner that only one of the gates is opened thereby at any instant irrespective of the voltages applied to the matrix from the input control circuits.

15. A transmitter according to claim 13, comprising a plurality of said sources each providing signals in the form of selections of pulses from a group, and a distributor to connect said sources in turn to said-information converting means, whereby multi-channe'l communication can be established.

16. A transmitter according to claim 15, wherein each of the said sources comprises an automatic sender for generating teleprinter signals, each teleprinter signal be ing generated .by a selection from "a 'grou'p'of five pulses, and wherein means are provided to synchronise said senders with said distributor to generate said teleprinter signals in turn.

17. A transmitter according to claim 10, comprising a modulator and connections to apply the bursts of oscillations to the modulator to produce frequency shift of a radio-frequency carrier.

18. A receiver for use with a'transmitter according to claim 10, comprising means to generate a plurality of pulses proportional to the number of cycles of'ea'ch transmitted and received burst of oscillation received Within a predetermined time interval and in response to each plurality of pulses so generated to reproduce the information which the burst represents.

19. A receiver for use with a transmitter according to claim 13 comprising means to generate a plurality of working pulses proportional to the number of cycles of each transmitted and received burst of oscillation receivedwithin a predetermined time interval and in response to each plurality of pulses so generated to reproduce the pulse selection which the burst of oscillation represents. V

20. A receiver according to claim 19, wherein said means comprises a pulse forming circuit to produce a burst of working pulses in response to each received burst of oscillation, a primary gate device, a pulse counter, means to apply each burst of working pulses to said counter by way of said primary gate, and a control circuit to open said primary gate only during a centre part of each burst of Working pulses. f j

21. A receiver according to'claim 20, Wherein't'he con trol circuit comprises a second gate, means for applying to the input terminal of the second gate timing pulses which occur between successive ones of the working pulses, a synchronizing circuit adapted to open the second gate to allow a timing pulse to passtherethrough at an instant after the commencement of a burst of working pulses, a clock device adapted toprovide, in response to "a timing pulse passed by the second gate, a gating pulse which, in operation, is applied to open the primary gate and has a duration such that the primary gate closes before the end of the last named burst of working pulses and between two working pulses. v

22. An electric signalling system.comprising'nieansior presenting information to be transmitted in a'one out-ofm pulse code, means for transmitting in succession bursts of oscillations, the frequencies of which are proportional to the numbers which successively presented coded'pulse's represent in said code, means for receivingl'sa'id bursts io'f oscillation, means to generate a plurality of pulses proportional to the number of cycles of each received burst cillations, the frequencies of which are proportional to the numbers which successively presented coded pulses represent in said code, means for receiving said bursts of oscillations, and means to generate a plurality of working pulses proportional to the number of cycles of each received burst of oscillation received within a predetermined time interval and in response to each plurality of pulses so generated to reproduce the pulse se- 'lection which the burst of oscillation "represents.

'24. An electric signalling system for signalling in formation which appears in the form of different selections of pulses from a group of pulses, the system comprising means to generate a plurality m of oscillations of predetermined different frequencies, :1 correspond- "ing plurality'm of gate devices,jm'e'ans 'to' apply said oscillations to said gate devices respectively,"a common output terminal for said gate devices, a rectifier matrix having a number of input control circuits for said matrix equal to the total number of pulses in the said group, a plurality m of output terminals on said matrix, means connecting the said output terminals to the gate devices respectively to control the opening and closing thereof, the input control circuits being responsive to the said selections to apply control voltages to the matrix which is so arranged that only one of the gate devices is opened thereby at any instant irrespective of the voltages applied to the matrix from the input control circuits and the frequencies of the oscillations being so chosen that each burst of oscillation transmitted by an opened gate device is proportional to the number of the gate device, means to transmit successive bursts of the oscillations appearing at'the common output terminal of the gate devices, receiving means comprising a pulse forming circuit to produce a burst of working pulses in response to each received burst of oscillation, a primary gate device, a pulse counter, means to apply each burst of working pulses to said counter by way ofsaid primary gate, a second gate, means for applying to the second gate timing pulses which occur between successive ones of the working pulses, a synchronizing circuit adapted to open the second gate to allow a'timing pulse to pass therethrough at an instant after the commencement o f a burst of working pulses, a clock device adapted to provide, in response to a timing pulse passed by the second gate, a gating pulse which, in operation,is applied to openthe primary gate device and has adur'ation such that the primary gate device closes before the end of the last-named burst of working pulses and between two working pulses, and an output circuit connected to'the counter to reproduce the pulse selections which the received bursts of oscillation represent.

. (References on followingpage) References Cited in the file of this patent UNITED STATES PATENTS Hammond Dec. 30, 1919 Chireix et a1. July 16, 1935 Botgeson Dec. 29, 1936 Cox Nov. 10, 1942 Chapin July 1, 1947 Crosby July 27, 1948 22 2,495,705 Devaux Ian. 31, 1950 2,570,716 Rochester Oct. 9, 1951 2,833,857 Robin May 6, 1958 OTHER REFERENCES Publication: Rectifier Networks for Multiposition Switching, by D. R. Brown and N. Rochester in Proceedings of the I.R.E." of February 1949, pages 139 10 147. 

